1. Field of the Invention
The field of the invention is methods and apparatus for improving the production of cement from raw materials having high alkali chlorides and sulfates. More specifically, the method and apparatus of the present invention are directed to preheating of dry raw meal containing high alkali chloride and sulfate prior to introducing the meal into a long dry process rotary kiln.
2. Description of the Prior Art
Rotary kilns equipped with suspension cyclone preheater systems have gained substantial popularity in the manufacture of cement from raw meal since their introduction in the early 1950's. Many such systems utilize a standard four stage cyclone preheater kiln, which are noted for their high productivity and thermal efficiency. However, cement raw materials which are high in alkali chlorides and sulfates are not suitable for use in standard four stage cyclone preheater kilns. Volatilized alkali salts, particularly chlorides, carried by dust laden gases leaving the rotary kiln, condense in the lower temperature cyclone preheater system. This results in choking and jamming of the system due to a build-up of crust-forming deposits, particularly in the fourth stage cyclone preheater, where the temperature is approximately 820.degree. C. The problem is aggravated by the inherent increase in alkali concentrations in the preheater system as a result of internal recycling. In addition, dust with high alkali content from dust collectors cannot be recycled and must be rejected.
The alkali chlorides volatilize before reaching sintering zone temperatures, and greatly enhance the possibility of forming crust deposits in the system. Permissible maximum chloride (Cl) content in raw meal is required to be as low as 0.05%. Typical maximum limits for total alkali (expressed as Na.sub.2 O) and of sulfate (SO.sub.3) are 1.5% and 1% respectively. Where chloride contents are marginally higher, it is possible to reduce problems of choking by partially by passing kiln exit gasses. In addition, single or two-stage preheater systems with lower thermal efficiencies are less prone to choking and jamming problems.
Alkali encrustation has also been observed as a problem in other preheater cement production systems, such as those which utilize grate preheaters.
When cement is produced from raw meal having higher alkali chloride content, a long dry process kiln, with or without heat exchange chains, is commonly preferred, although this system is known to be less thermally efficient than four stage cyclone preheater kilns. Use of long dry process kilns substantially reduces problems resulting from alkali recycling and build up of encrustations in the kiln. However, to adapt present long dry process kiln systems to produce cement from high alkali and sulfate containing materials, modifications in operating practice are required.
These modifications include:
(a) maintaining a high exit gas temperature (over 500.degree. C.) to prevent alkali chloride condensation and recycling in the kiln;
(b) avoidance of cyclone type dust collections with constricted sections that are susceptible to choking and jamming;
(c) use of cold air for cooling kiln exit gases to below approximately 400.degree. C. to avoid damaging the waste gas fan; and
(d) rejection of all or part of the collected alkali-rich dust to avoid build-up of chloride concentration in feed to the kiln.
In certain systems, where high alkali raw meal is processed, long dry process kilns can be operated on natural draft to avoid problems of cooling of high temperature exit gases. Otherwise cooling of exit gases is necessary to safe guard waste gas fans. The above pattern of operating precautions can be considerably relaxed for cement plants with relatively lower alkali chloride contents. Outputs of long dry process kilns are up to 2,000 tpd.
Heretofore several attempts have been made to increase the production of rotary cement kilns in terms of the tons of cement clinker produced, and to improve the thermal efficiency of the kiln. Examples of prior art methods and apparatus for making cement are disclosed in the following patents:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 3,895,955 Kondo, et al. 3,986,886 Sylvest ______________________________________
The Kondo patent discloses the use of waste gases in stages of a cyclone preheater system for controlling the operating temperature in a rotary kiln, and utilizing a fluidized calcinator having a separate heat source to increase the capacity of the rotary kiln, whereby the kiln can be operated continuously and stably for long periods of time.
The Sylvest patent teaches drying of a raw meal slurry utilizing hot gases from a rotary kiln and a separate heater for providing supplementary hot gases to various stages of a multistage cyclone preheater system.
As will be described in greater detail hereinafter, the method and apparatus of the present invention differ from the teachings ofthese prior art patents by providing a separate combustion chamber for preheating dry raw meal containing high alkali chloride and sulfate quantities prior to feeding the meal to a single stage cyclone separator and a long dry process rotary kiln. Moreover, the method and apparatus of the present invention can be utilized with or without a vibratory deaerator after the cyclone separator to reduce entrainment of air in the raw meal feed.